1.0 Field of the Invention
The invention relates to a method of operating a dosing pump, which is driven by an asynchronous motor, with a pump drive, which converts the motor revolutions into pump strokes consisting of a pump suction cycle and pump pressure cycle and having a defined stroke frequency, wherein continuous pump strokes are carried out during a dosing phase.
2.0 Discussion of the Related Art
Various forms of dosing pumps driven by an electric motor are used for dosing liquids of the most diverse kinds in precise quantities. From a motor power of about 40 watts, the preferred use for such dosing pumps is asynchronous motors supplied by a 230 volt or 115 volt standard operating mains with alternating voltage and alternating current at a mains frequency of 50 or 60 hertz. As long as a mains voltage of 230 volts and the mains frequency of 50 or 60 hertz are applied to the asynchronous motor of these dosing pumps, the asynchronous motors run at a load-dependent, almost constant rotational speed. The motor rotational speed is converted by way of a transmission arrangement into pump strokes performed by a pump element, for example a piston or a diaphragm, producing the respective pump suction and pressure cycles. In the case of a dosing pump driven by an asynchronous motor with 230 volts and 50 or 60 hertz, the maximum stroke frequency, which is predetermined by virtue of the transmission arrangement, is usually between 120 and 180 strokes per minute. Each stroke consists of one suction cycle and one pressure cycle of the pump. Electrical drive control signals, which allow the asynchronous motor to execute a respective stroke of the pump element, for example diaphragm or piston, are supplied to the asynchronous motor by a so-termed water meter or a standard signal transmitter or an internal cycle transmitter. The drive control signals are repeated until the number of strokes carried out for the desired dosing quantity has been performed. A dosing phase of the pump is composed of this number of strokes. A dosing phase is triggered by an electrical start signal supplied to the dosing pump.
In these pumps an alternating voltage with constant frequency is applied during each stroke consisting of a suction cycle and pressure cycle, so that the suction cycle and pressure cycle demand the same period of time. This has the consequence that the product in the dosing duct connected to the pump is conveyed for the time corresponding to the respective pressure cycle, and a standstill phase or xe2x80x9cdosing gapxe2x80x9d subsequently arises in the dosing duct for the same length of time of the suction cycle before further product is conveyed in the dosing duct by a new pressure cycle. This can lead to an unsatisfactory conveying of product in the dosing duct.
This problem is even more serious in cases where the pump is to dose at a stroke frequency lower than the maximum possible frequency. This occurs when by the asynchronous motor is initially being switched on by means of a drive control signal for a complete stroke consisting of a suction cycle and pressure cycle, but subsequently remains switched off for the duration of a time period necessary for achieving the desired stroke frequency before a new stroke is started by a new drive control signal. An even more unfavourable distribution of the product, which is to be dosed, in the dosing duct results from this so-termed pulse/pause drive control, and so-termed dosing clouds arise at intervals of greater or lesser length.
Another possibility of reducing the stroke frequency consists controlling the asynchronous motor in drive by way of a frequency changer, which supplies to the motor an alternating voltage frequency or alternating current frequency lowered by comparison with the mains frequency of 50 or 60 Hertz. This has the consequence that the motor rotational speed and thus the stroke frequency of the pump are reduced. With the lowered frequency, the time duration of the suction cycle and pressure cycle and thus the stroke frequency are prolonged due to the lower motor rotational speed. The suction cycle and pressure cycle are, however, still of equal length, which means of the same duration in time. The advantage relative to the first method is that due to the prolonged cycle times a pause drive control during which the motor is stopped is no longer necessary for achieving the desired stroke frequency. The pressure cycle is prolonged relative to a pulse/pause drive control at the same stroke frequency, so that a better distribution of the product, which is to be dosed, in the dosing duct is established. However, the suction cycle is also drawn out to the same degree, as a result of which the problem of large gaps without product, which is to be dosed, in the dosing duct still arises.
With the problems of the prior art in mind, an object of the invention is to improve; and dosing performance during operation of dosing pumps with an asynchronous motor drive.
In one embodiment of the invention, this object is met by application, in each pump stroke, to the asynchronous motor of an electrical alternating voltage at higher frequency during the pump suction cycle, and the same electrical alternating voltage at lower frequency relative to that in the pump suction cycle during the pump pressure cycle. The possibility is thus created by the invention of structuring the length or duration in time of the suction cycle and pressure cycle of a stroke to be different. The higher the frequency applied to the asynchronous motor during the suction cycle, the faster the motor turns and the shorter the suction cycle. On the other hand, the lower the frequency, the longer the pressure cycle. It is thus possible to significantly shorten the suction cycle relative to the pressure cycle in its length or duration in time. If a shortest possible suction cycle and a longest possible pressure cycle are desired, the disadvantageous xe2x80x9cdosing gapsxe2x80x9d in the state of the art no longer arise. The length of the suction cycle is minimized, and thus the time during which no product in a dosing duct is dosed is kept as short as possible, by application of the higher frequency during the suction cycle. The suction cycle is then adjoined by the pressure cycle. This can be regulated in its duration or length in terms of time by application of an appropriate lower alternating voltage frequency to the asynchronous motor, so that a time duration for each stroke consisting of a suction cycle and pressure cycle results, which duration corresponds to the desired stroke frequency. The pressure cycle is, through application of the lower frequency, arranged to be as long in time as possible having regard to the predetermined stroke frequency, i.e. it is maximized in terms of time.
Thus an almost constant dosing of the product in a dosing duct is possible by the invention, with interruption merely by short gaps during the suction cycle. In addition, a further advantage by comparison with pulse/pause control is that due to the regulable frequency during the pressure cycle, the length or duration thereof in time can be set, in particular, independently of the suction cycle and thus the desired stroke frequency can be achieved. As pause times no longer arise during which the asynchronous motor is stopped, the pump drive is mechanically treated in a more gentle manner. By contrast to pulse/pause control it is no longer exposed to any shock loadings, whereby the service life of the drive is increased, particularly in the case of a higher pump output.
The invention thus generally provide that the pump suction cycle and pump pressure cycle are regulated in a different manner with respect to their duration or length in time whereby they are controllably different. This contrasts with the state of the art, in which the suction cycle and pressure cycle are arranged to be of equal length.
It is advantageous if a frequency (typically 50 Hertz or 60 Hertz in the US), above the frequency of a usual 230 or 115 volt standard operating mains is applied as the higher frequency and a frequency below the frequency of a usual 230 or 115 volt standard operating mains is applied as the lower frequency, in one embodiment of the invention.
It is furthermore of advantage in another embodiment of the invention if, for the control and regulation of the length of the suction cycle and pressure cycle, a frequency change is carried out at the start of each pump suction cycle and pump pressure cycle.
In a preferred embodiment of the invention, for achieving a particularly favourable and technically low-cost realization of the method according to the invention, that the settings of a pump element, which produces the pump suction process and pump pressure process of the dosing pump, are detected at the forward dead centre thereof indicative of the start of a pump suction cycle and at the rearward dead centre thereof indicative of the start of a pump pressure cycle by means of position sensors, and that electrical position signals, which are processed into electrical drive control signals triggering the respective frequency change, are transmitted by these position sensors at the respective dead centre.
In a preferred embodiment of the invention, it is particularly advantageous if the position signals are fed to a control unit, and are processed by this unit into the drive control signals triggering the respective frequency change.
It is of advantage to provide a frequency changer for the setting and regulation of the desired frequencies. In a refinement, the invention therefore further provides that the drive control signals are supplied to a frequency changer by which the asynchronous motor is driven by voltages with the respective frequency.
The reaching of the forward and rearward dead centre of the dosing pump element can be detected by reference to the rotor setting of the asynchronous motor or the eccentric setting of a transmission. Accordingly, in another embodiment of the invention the forward and rearward dead centre are detected by reference to the rotor setting of the asynchronous motor or the eccentric setting of a transmission.
In yet another embodiment of the invention, for the triggering of a dosing phase, i.e. a number of pump strokes corresponding to the volume to be dosed by the dosing pump, an electrical start signal triggering the dosing phase is supplied to the control unit when the pump element is positioned in its forward or rearward dead centre.
The pump frequency or stroke frequency is preferably regulated in such a manner that a number of pump strokes corresponding to the volume to be dosed is performed during a dosing phase.
In order to keep the mechanical loading of the dosing pump within manageable limits, and thus the constructional cost within a manageable frame, the invention further provides that the pump strokes are executed at a stroke frequency between 10 and 180 strokes per minute.
It is of advantage both for the mechanical loading of the dosing pump, and for the control and regulation outlay to be expended, if the stroke frequency is constant during a dosing phase, which the invention thus equally proposes.
In a further embodiment the invention, and delete xe2x80x9cprovides on the one hand thatxe2x80x9d provides on the one hand that the individual suction cycles during a dosing phase are arranged to be of equal length and on the other hand that the individual pressure cycles during a dosing phase are arranged to be of equal length. This has the advantage of a uniform mechanical loading of the dosing pump.
In another preferred embodiment of the invention, for the regulation of different dosing outputs, that the length of a suction cycle is preset for a maximum stroke frequency, or a 100 percent dosing output, and the length of a pressure cycle is set or regulated to be a complementary value necessary for achieving the respective actual dosing output or stroke frequency. Whereas the length of each suction cycle is designed for the maximum stroke frequency at 100 percent dosing output, and is set by an appropriate frequency supplied to the asynchronous motor, as well as remains constant independently of the respective actual dosing output, the length of each pressure cycle is regulated in dependence on the respective actual dosing output and the stroke frequency connected therewith by supply of a corresponding frequency to the asynchronous motor.
The foregoing object is met in accordance with the invention in such a way that, for a constant operating mains voltage, the frequency changer supplies to the asynchronous motor in each pump stroke alternating current or voltage at higher frequency during a pump suction cycle, and alternating current or voltage at a lower frequency compared with that of the pump suction cycle during a pump pressure cycle.
The method according to the invention can, in this manner, be realized via a dosing pump in a technically relatively simple manner. This dosing pump similarly possesses the foregoing advantages expressed in relation to the method.
In another embodiment of the invention it is similarly provided in the case of the dosing pump that the higher frequency driven voltage lies above the frequency of a usual 230 or 115 volt standard operating mains, and the lower frequency lies below the frequency of a usual 230 or 115 volt standard operating mains.
It is of advantage, and is provided by the invention in one embodiment, for the regulation and drive control of the suction cycle, if the frequency changer changes over to the lower frequency at each rearward dead centre, which represents the start of a pump pressure cycle, of a pump element producing the suction and pressure processes of the dosing pump, and changes over to the higher frequency at each forward dead centre thereof, which represents the start of a pump suction cycle.
In order to trigger the respective frequency changeover, and to be able to realize this in a technically relatively simple manner, it is moreover of advantage if electrical control signals supplied by the control unit to the frequency changer trigger the respective frequency change, which the invention thus equally proposes.
In addition, in another embodiment provided by the invention, it is particularly useful and advantageous for the realization of the frequency change, if the control unit is associated with position sensors detecting the forward and rearward dead centre of the pump element, and supplying electrical position signals to the control unit when the pump element is positioned in each of its dead centres.
In an advantageous embodiment, the invention then proposes that the control unit processes the position signals into the respective drive control signals.
A particularly favourable possibility for detection of the forward and rearward dead centre consists in the position sensors detecting the forward and rearward dead centre of the pump element by reference to the rotor setting of the asynchronous motor or the eccentric setting of a gear, which the invention thus equally proposes.
Moreover, for triggering of a dosing phase the invention provides that an electrical start signal supplied to the control unit triggers a dosing phase.
In a further embodiment the invention provides on the one hand that the individual suction cycles are arranged to be of equal length during a dosing phase and on the other hand that the individual pressure cycles are arranged to be of equal length during a dosing phase.
According to a further development it is of advantage for the regulation and control of the dosing pump if the length of each suction cycle is directed to the maximum number of pump strokes that can be carried out for a 100 percent dosing output and the length of each pressure cycle results as a complementary value necessary for achieving the respective actual dosing output.
The dosing pump has in its individual refinements and developments the same advantages as expressed in the foregoing for the method.